The electronic band structure of GaBiAs/GaAs layers: Influence of strain and band anti-crossing

The GaBi_xAs_1−x bismide III-V semiconductor system remains a relatively underexplored alloy particularly with regards to its detailed electronic band structure. Of particular importance to understanding the physics of this system is how the bandgap energy E_g and spin-orbit splitting energy Δ_o vary relative to one another as a function of Bi content, since in this alloy it becomes possible for Δ_o to exceed E_g for higher Bi fractions, which occurrence would have important implications for minimising non-radiative Auger recombination losses in such structures. However, this situation had not so far been realised in this system. Here, we study a set of epitaxial layers of GaBi_xAs_1−x (2.3% ≤ x ≤ 10.4%), of thickness 30–40 nm, grown compressively strained onto GaAs (100) substrates. Using room temperature photomodulated reflectance, we observe a reduction in E_g, together with an increase in Δ_o, with increasing Bi content. In these strained samples, it is found that the transition energy between the conduction and heavy-hole valence band edges is equal with that between the heavy-hole and spin-orbit split-off valence band edges at ∼9.0 ± 0.2% Bi. Furthermore, we observe that the strained valence band heavy-hole/light-hole splitting increases with Bi fraction at a rate of ∼15 (±1) meV/Bi%, from which we are able to deduce the shear deformation potential. By application of an iterative strain theory, we decouple the strain effects from our experimental measurements and deduce E_g and Δ_o of free standing GaBiAs; we find that Δ_o indeed does come into resonance with E_g at ∼10.5 ± 0.2% Bi. We also conclude that the conduction/valence band alignment of dilute-Bi GaBiAs on GaAs is most likely to be type-I.